Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Trigraphy - Photo editing app that allows you to make mosaic/abstract looking art.
Pixilart - A drawing app for delightfully retro pixel art
GIF Brewery by Gfycat - Create stunning GIFs from videos & screen recording for free
Pixel Art Studio - A purpose-built drawing tool for pixel artists
WidsMob Montage - Make stunning mosaic photos or montage shape with tiles, which supports both JPEG and PNG files.